Two projects are proposed. The first deals with the involvement of selenium with the hepatic microsomal cytochrome P-450 system. Published and unpublished work has shown a dependence of the normal response of cytochrome P-450 to phenobarbital on adequate dietary selenium. Studies of other components and of several functions of this system are being carried out to more precisely define the role of selenium. The second project is concerned with the amelioration of inorganic mercury poisoning by selenium. A selenium-mercury-containing plasma protein has been found in the rat. Current investigations are directed toward the purification of this protein and its physiological properties, i.e., the fate of mercury once it has become bound to this protein.